A dynamic random access memory (DRAM) is used as the memory device of a computer. Progress has been made in the size reduction and integration of the devices with the improvement in performance. At present, highly dielectric materials and electrode materials, which are important for promoting integration, are being developed.
When a dielectric layer such as a silicon oxide substrate and an electrode layer of, for example, copper or silver are laminated to each other, in order to prevent mutual bleeding of the materials from both layer, an iridium film or a platinum film is disposed as an intermediate layer. However, iridium is expensive, and platinum is difficult to etch, making it difficult to form an electrode structure. Accordingly, a ruthenium film and a ruthenium oxide film, which are inexpensive and easy to etch, are expected as intermediate layer suitable for DRAMs. Furthermore, in order to correspond to miniaturization of device structure with the improvement in performance of the DRAM, a film-forming process by a CVD method, instead of conventional sputtering, and development of a ruthenium material therefor are expected.
A liquid material is superior to a solid material for producing an excellent film by the CVD method. When a solid is used as a raw material, the composition of gaseous phase is heterogenous, preventing the thickness of a deposited film from being uniform. On the other hand, when a liquid is used as a raw material, the composition of gaseous phase is homogenous, allowing the thickness of a deposited film to be uniform. Since a precise supply of a raw material is possible by using a liquid as raw material and using a mass flow controller, film-forming conditions can be strictly controlled to more precisely control the thickness.
FIG. 1 shows a schematic diagram illustrating a bubbler method using a mass flow controller, as an example of the CVD method.
As ruthenium-containing CVD materials, for example, cyclopentadienyl-based materials, such as biscyclopentadienyl ruthenium and bisethylcyclopentadienyl ruthenium, and ruthenium complexes having amidinium as the ligands have been reported.
Since the biscyclopentadienyl ruthenium is a solid at room temperature, it is necessary to use a sublimation method for vaporizing it, and it is difficult to stably supply a sufficient amount of its vapor with high accuracy.
The bisethylcyclopentadienyl ruthenium is a liquid at room temperature and is possible to perform mass flow control by, for example, a bubbler method. However, high step coverage has not been obtained in a film-forming test using the CVD method. Furthermore, the material is unstable against air and therefore has a problem of difficulty in handling.
The ruthenium complex having amidinium as the ligand has been reported as a ruthenium complex showing high step coverage. It is reported that a ruthenium thin film can be uniformly formed down to the terminal portion of a fine pore with a radius of 200 nm and a depth of 8000 nm (aspect ratio: 40) by using this compound (see Non Patent Literature 1), and this is attracting attention as a material that can be applied to a process of producing an electrode of a DRAM/capacitor portion having a complicated structure. However, this compound is a solid at ordinary temperature and has a high boiling point of 203 to 205° C. (see Non Patent Literature 2). Therefore, a vaporizer for liquid cannot be used for forming a film by this compound.